This invention relates to waterless planographic-printing. More particularly it relates to novel photo-sensitive sheet constructions suitable for use in planographic printing wherein conventional fountain solutions are not necessary.
In planographic printing, as the name implies, both image and nonimage areas lie substantially in the same plane, e.g. the ink-receptive image areas are not subtantially raised from the surface of the printing plate. The nonimage or background areas are made ink-repellent so that upon ink application to the plate surface, only image areas accept the ink for subsequent transfer to sheet material.
Conventional lithography, the best known form of planography, generally requires a dampening of the printing plate with an aqueous fountain solution to effectively wet the background plate areas after which ink is rolled over the plate. The oily ink selectively wets the oleophilic image areas but is repelled from the dampened background areas.
To overcome difficulties inherent with conventional lithography, planographic printing plates not requiring dampening have recently been developed. These plates require only an inking system to be operative, and inherently contain ink repellent non-image or background areas. This relatively new concept has come to be known by the term driography, and plates useful therein have been termed driographic plates. Such a printing plate is disclosed in U.S. Pat. No. 3,511,178.
These plates generally operate on the principle that the background surface areas have sufficiently low adhesion to driographic printing inks that ink applied to the plate by an inking roller will not split away or transfer from the roller to the plate in such areas. A surface exhibiting such characteristics has been termed "abhesive".
At present, one technique for providing ink-receptive image areas on driographic printing plates is by selective removal of the abhesive coating. For example, in the aforementioned U.S. Pat. No. 3,511,178 a material which is abhesive, e.g. a curable silicone elastomer, is coated over a light-sensitive diazo resin layer on a suitable substrate. Development of the imaged plate removes the decomposed diazo resin and silicone elastomer overcoat in an imagewise fashion.
Another imaging technique is to overcoat an abhesive material with an ink receptive composition which after imaging can be selectively removed in an imagewise fashion with a developer. In this instance it is extremely difficult to obtain a satisfactory bond between the abhesive layer and the exposed overlayer.
In these prior art concepts it is difficult to obtain a driographic plate having a durable ink receptive image area. This is because the inherent characteristics of the abhesive material provide a surface to which it is difficult to bond an ink receptive layer. This difficulty of attaining a high degree of image adherence generally results in a plate having a substantially limited press life.
It has now been found that image adhesion can be greatly enhanced by providing an abhesive layer which will chemically bond to an overlying ink receptive layer in areas exposed to actinic radiation. This photobonding occurs because the abhesive layer contains therein ethylenically unsaturated groups which are copolymerizable with an overlayer of ethylenically-unsaturated free radical initiated photopolymerizable material. Te photopolymerizable overlayer, upon exposure, provides an oleophilic surface which is photobonded to the underlying abhesive layer upon exposure, thereby providing a durable ink receptive image area and correspondingly a press life superior to prior driographic printing plates.